Aldaric acids are a group of sugar acids, where the terminal hydroxyl or aldehyde groups of the sugars have been replaced by terminal carboxylic acids. These acids are characterized by the formula HOOC—(CHOH)n—COOH, with n being an integer of from 1 to 5. These dicarboxylic acids, on account of their combined functionalities, are interesting chemicals. E.g., as sequestering agents, corrosion inhibitors or monomers for making polymers made on the basis of dicarboxylic acids, such as polyesters or polyamides. Preferred aldaric acids are those wherein n is an integer of from 3 to 5. Aldaric acids of particular interest are those derived from C5 and C6 sugars, like xylaric acid, glucaric acid, mannaric acid, gularic acid and iduronic acid. An aldaric acid of particular interest is galactaric acid, the aldaric acid corresponding to the sugar galactose. Applications for galactaric acid range from sequestering agents (Kohn et al. Collect. Czech. Chem. Commun. 1986, 1150) to building blocks for polymers (e.g. Moore & Bunting Polym. Sci. Technol., Adv. Polym. Synth., 51). Other aldaric acids of particular interest are glucaric acid, mannaric acid and gularic acid, aldaric acids that can be obtained from the carbohydrate fraction of biomass sources including pectins and a variety of different seaweeds.
Aldaric acids can be prepared by the oxidation of the corresponding aldoses into the aldaric acids. To this end in literature the nitric acid oxidation of aldoses into aldaric acid is well known, as well as the TEMPO mediated oxidation of aldoses into aldaric acids. However, for these procedures a double oxidation of both the aldehyde group as well as the terminal hydroxyl group is required resulting in relatively low yields, and numerous side-products.
The background art includes some examples on the oxidation of galacturonic acid to galactaric acid. According to FR 2699937 galacturonic acid can be oxidized to galactaric acid using an electrochemical cell giving galactaric acid in 80% yield (90% conversion, 90% selective) after 7 h at 55-58° C., in 1M concentration. Rangappa et al. (J. Carbohydr. Chem. 1997, 359) reported the oxidation of galacturonic acid by using excess sodium N-chlorobenzenesulfonamide in alkaline medium, while Shashikala and Rangappa (J. Carbohydr. Chem. 2002, 491) reported the oxidation of galacturonic acid by using excess sodium N-bromoarylsulfonamides in alkaline medium. According to WO 2010/072902 via a microbial host strain expressing uronate dehydrogenase enzyme (EC 1.1.1.203), galacturonic acid is converted into galactaric acid. Here a typical incubation time is 3 to 5 days to convert 1-2 wt % of galacturonic acid into galactaric acid (mucic acid) without mentioning selectivity or isolated yield.
The oxidation of glucuronic acid is described in U.S. Pat. No. 6,518,419 by using peracids as an oxidant. Alternatively hydrogen peroxide can be used to form the peracid in situ. In this procedure TEMPO in combination with a halide, preferably a bromide, act as the catalyst. D-glucaric acid was isolated as the K-salt in 62% yield.
In US 20080187984 the oxidation of glucuronic acid to glucaric acid is performed via enzymatic procedures. Three different polypeptides are suggested: peptides with non-specific hexose oxidase activity (EC 1.1.3.5), peptides with aldehyde dehydrogenase [NAD(P)] activity (EC 1.2.1.5, EC 1.2.1.3 (NAD), EC 1.2.1.4 (NADP) or by a polypeptide having aldehyde oxidase activity (EC 1.2.3.1). No isolated yields are reported.
In the same patent US 20080187984 it is suggested that the oxidation can be performed via a chemical step by using molecular oxygen and a catalyst. The patent includes an example where glucuronic acid is oxidized with molecular oxygen and a 5% palladium on carbon catalyst. This procedure requires high catalyst loadings (10 g Pd/C for the conversion of 5 g glucuronic acid) in order to obtain reasonable selectivity's. A product yield of 90% was reported based on HPLC analysis. This work was based upon the work described in U.S. Pat. No. 5,817,870, were it was shown that high metal loadings (>10 percent by weight) improve the selectivity for oxidation reactions.
None of these methods is attractive for commercial production. The background also includes examples of the oxidation of aldoses to the corresponding aldonic acids over Au based catalysts. U.S. Pat. No. 7,892,031 describes the oxidation of aldoses like glucose and lactose over Au/TiO2 catalysts at 40 C and pH 9 in high selectivity. The oxidation of alduronic acids into aldaric acids is not described.
Biorefineries serve to conduct the sustainable processing of biomass into a spectrum of marketable biobased products and bioenergy. A biorefinery is an installation that can process biomass into multiple products using an array of processing technologies. In general, biomass coming from plants, will result in streams based on lignin, cellulose, and hemicellulose, respectively. Hemicelluloses can be removed from biomass, e.g. by treatment with hot pressurized water. This results in formation of water soluble oligomeric and monomeric sugars and their dehydration products such as furfural and hydroxymethyl furfural. Another source of hemicelluloses is in the agro-food industry. Whilst hemicelluloses, in theory, are a source of a wide variety of useful chemicals, it is desired to find methods to make better use of this potential, by providing economically attractive processes to harvest such chemicals therefrom. A particular interesting hemicellulosic feedstock from the agro-food industry comprises sugar beet pulp, a byproduct of the sugar beet industry. Sugar beet pulp contains a high content of pectic substances, being composed of arabinose and galacturonic acid as the main monomers. Other sources of pectins are alls kind of different fruits, including e.g. apples, carrots, cherries and citrus fruits, especially citrus peels. Another potential source of uronic acids is being formed by alginates. A large variety of seaweeds including red and brown seaweeds like Laminaria digitata, Saccharina latissima and Ulva lactuca contain huge amounts of alginates being composed of mannuronic acid and guluronic acid as the composing monomers; after hydrolysis of the alginates such monomers can be used as feedstock for the production of aldaric acids.
It would be desired to provide a process enabling the unlocking of the chemical potential present in the form of uronic acids in hemicellulosic streams.